1. Field of the Invention
This invention relates to systems which use optical Fourier computing to detect and classify defects in patterns.
2. Description of Related Art
Inspecting a patterned device is typically performed in one of three ways: (1) manually inspecting the device with the naked eye or with some enlarging lens or microscope; (2) digitizing an image of the device and digitally processing the image to detect defects; and (3) optically decomposing an image of the device with a Fourier transforming lens which forms a Fourier transform of the image, and a spatial filter which blocks light expected from a defect-free device and transmits light which would indicate a defect. Frequently, systems which optically form a Fourier transform also form an image from the light transmitted by the spatial filter, digitize the image, and perform defect characterization by digital computing.
Fourier optical inspection for defects is known in the art and described for example in: U.S. Pat. No. 4,330,205, to Murakami et al., for inspecting flat surfaces; U.S. Pat. No. 4,330,775, to Iwamoto et al., for inspecting periodic patterned surfaces; U.S. Pat. No. 4,806,774, to Lin et al., for inspecting integrated circuit patterns on wafers; U.S. Pat. No. 5,098,191, to Noguchi et al., for inspecting mask reticles. All these patents discuss different aspects of Fourier optical inspection for defect detection.
U.S. Pat. No. 5,264,912, to Vaught et al., describes broad band light sources for the purpose of Fourier optical inspection.
Many spatial filter designs are also known in the art. Specifically U.S. Pat. No. 4,929,081, to Yamamoto et al., describes an erasable optical space modulator; U.S. Pat. No. 5,172,000, to Scheff et al., describes a standard spatial filter for defect detection; U.S. Pat. No. 5,276,498, to Galbraith et al., describes an adaptive spatial filter based on liquid crystal light modulator used in a scanning detector system; and U.S. Pat. No. 5,289,260, to Miyazaki et al. describes a controlled filter spot on a photosensitive plate.
All of the above referenced patents are incorporated by reference herein in their entirety.
The prior art, including above referenced patents, have used spatial filters which block a first portion of light generated by the optics, and transmit a second portion of the light. The second portion of the light is of interest, and typically corresponds to light generated by defects. However, by blocking the first portion of light, information about a device being tested is lost, which can make some defects undetectable or make characterization of a defect more difficult. Accordingly, detecting and characterizing defects may require time consuming additional testing of the device.